That is good advice - to check with the galvanizers themselves, so you will know the answer first hand.
Speaking from experience, you should find that rust is no problem for galvanizing process. (Talking about normal surface rusting, not severely corroded steel) Heavy lacquer that is often found on pipe, or painted steel presents more of a problem.
Basically, the galvanizer will dip the object to be galvanized into a caustic bath to remove grease and oil. Some paint will be dissolved as well but heavy accumulations may not come off completely without extra time and effort.
Then the object goes to an acid bath for pickling. This step removes rust and mill scale, and leaves a bare steel surface.
Somewhere along the line, fluxing happens to prevent oxidation before dipping into the zinc bath.
If you did clean off the rust first, the galvanizer would still have to follow the same steps.
Chet Guilford
Around this area, the galvanizer is responsible for surface preparation. As Chet described, they usually do some sort of caustic cleaning to remove organic matter such as dirt, paint, oil and grease, followed by an acid pickling to remove mill scale and rust, and then a fluxing to prevent oxides from forming prior to galvanizing.
I had worked at a plating company for many years, doing all different types. As mentioned paint and heavy grease should present the only problems (submerge or spray times could be effected with these items). Plating companies must use and good pre cleaning process to avoid contaminating the paint or plating bath and for the plating to adhere properly to the metal.
Remove any paint markings (ie. paint markers) from the material. The caustic bath or pickleing as we call it doesn't remove it very well and the paint will show thru.
Brian
I work for a company who has everything hot dip galvanized after fabrication. We have found a couple of brands and types of paint markers that will come off in the galv process, but you have to verify the marker is made for that purpose otherwise it will show up as a shadow under the galv. And believe me you can read what was marked on the item. One employer I worked for had to strip and redip some items because someone got cute and wrote things about someone else on them. You know how guys in the shop are.
We send items to our galvanizer that are very rusted and don't have any problems, you just might get a rougher finish surface. As stated though, paint and grease are the worst, they should be cleaned off first. Also you have to be concerned about the chemistry of your steel, there are certain limits of phos and sulpher you should not exceed for a good finished product. We had some steel that would not take the galv, come to find out it was the steel chemistry. We order all our steel within certain chemistry limits to avoid this issue.
Most galvanizers have a small pamphlet they will give you for free explaining most everything. Just call them and ask them to send you a copy.
Here, light to moderate corrosion, as is typically found in a steel yard, is no problem at the 2 galvanizers that we regularly use. As has been stated above, the laquers found on pipe and tubing is definitely a problem. Likewise, any paint or paint stick is removed. In our receiving yard and material prep depts, we only use Markal P paint sticks ( http://www.markal.com/ProductDetails.aspx?ProductID=86 ) so we don't have to worry about leaving paintstick on any pieces that may later be galvanized. They are about twice as expensive as regular (Markal B) paint sticks, but are harder and last twice as long. We still have to remove yellow paint markings from mills and vendors and even "laser" mill heats and size ids before galvanizing.
The above advise of checking with your galvanizer is sound advise as different acids are used at different galvanizers.
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